ShedLock

Apache License 2 Build Status Maven Central

ShedLock makes sure that your scheduled tasks are executed at most once at the same time. If a task is being executed on one node, it acquires a lock which prevents execution of the same task from another node (or thread). Please note, that if one task is already being executed on one node, execution on other nodes does not wait, it is simply skipped.

ShedLock uses external store like Mongo, JDBC database, Redis, Hazelcast, ZooKeeper or others for coordination.

Feedback and pull-requests welcome!

ShedLock is not a distributed scheduler

Please note that ShedLock is not and will never be full-fledged scheduler, it's just a lock. If you need a distributed scheduler, please use another project. ShedLock is designed to be used in situations where you have scheduled tasks that are not ready to be executed in parallel, but can be safely executed repeatedly. Moreover, the locks are time-based and ShedLock assumes that clocks on the nodes are synchronized.

Components

Shedlock consists of three parts

Usage

To use ShedLock, you do the following 1) Enable and configure Scheduled locking 2) Annotate your scheduled tasks 3) Configure a Lock Provider

Enable and configure Scheduled locking (Spring)

First of all, we have to import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-spring</artifactId>
    <version>4.12.0</version>
</dependency>

Now we need to integrate the library with Spring. In order to enable schedule locking use @EnableSchedulerLock annotation

@Configuration
@EnableScheduling
@EnableSchedulerLock(defaultLockAtMostFor = "10m")
class MySpringConfiguration {
    ...
}

Annotate your scheduled tasks

import net.javacrumbs.shedlock.spring.annotation.SchedulerLock;

...

@Scheduled(...)
@SchedulerLock(name = "scheduledTaskName")
public void scheduledTask() {
    // To assert that the lock is held (prevents misconfiguration errors)
    LockAssert.assertLocked();
    // do something
}

The @SchedulerLock annotation has several purposes. First of all, only annotated methods are locked, the library ignores all other scheduled tasks. You also have to specify the name for the lock. Only one task with the same name can be executed at the same time.

You can also set lockAtMostFor attribute which specifies how long the lock should be kept in case the executing node dies. This is just a fallback, under normal circumstances the lock is released as soon the tasks finishes. You have to set lockAtMostFor to a value which is much longer than normal execution time. If the task takes longer than lockAtMostFor the resulting behavior may be unpredictable (more then one process will effectively hold the lock).

If you do not specify lockAtMostFor in @SchedulerLock default value from @EnableSchedulerLock will be used.

Lastly, you can set lockAtLeastFor attribute which specifies minimum amount of time for which the lock should be kept. Its main purpose is to prevent execution from multiple nodes in case of really short tasks and clock difference between the nodes.

Example

Let's say you have a task which you execute every 15 minutes and which usually takes few minutes to run. Moreover, you want to execute it at most once per 15 minutes. In such case, you can configure it like this

import net.javacrumbs.shedlock.core.SchedulerLock;

@Scheduled(cron = "0 */15 * * * *")
@SchedulerLock(name = "scheduledTaskName", lockAtMostFor = "14m", lockAtLeastFor = "14m")
public void scheduledTask() {
    // do something
}

By setting lockAtMostFor we make sure that the lock is released even if the node dies and by setting lockAtLeastFor we make sure it's not executed more than once in fifteen minutes. Please note that lockAtMostFor is just a safety net for a case that the node executing the task dies, so set it to a time that is significantly larger than maximum estimated execution time. If the task takes longer than lockAtMostFor, it may be executed again and the results will be unpredictable (more processes will hold the lock).

Configure LockProvider

There are several implementations of LockProvider.

JdbcTemplate

First, create lock table (please note that name has to be primary key)

# MySQL, MariaDB
CREATE TABLE shedlock(name VARCHAR(64) NOT NULL, lock_until TIMESTAMP(3) NOT NULL,
    locked_at TIMESTAMP(3) NOT NULL DEFAULT CURRENT_TIMESTAMP(3), locked_by VARCHAR(255) NOT NULL, PRIMARY KEY (name));

# Postgres
CREATE TABLE shedlock(name VARCHAR(64) NOT NULL, lock_until TIMESTAMP NOT NULL,
    locked_at TIMESTAMP NOT NULL, locked_by VARCHAR(255) NOT NULL, PRIMARY KEY (name));

# Oracle
CREATE TABLE shedlock(name VARCHAR(64) NOT NULL, lock_until TIMESTAMP(3) NOT NULL,
    locked_at TIMESTAMP(3) NOT NULL, locked_by VARCHAR(255) NOT NULL, PRIMARY KEY (name));

# MS SQL
CREATE TABLE shedlock(name VARCHAR(64) NOT NULL, lock_until datetime2 NOT NULL,
    locked_at datetime2 NOT NULL, locked_by VARCHAR(255) NOT NULL, PRIMARY KEY (name));

# DB2
CREATE TABLE shedlock(name VARCHAR(64) NOT NULL PRIMARY KEY, lock_until TIMESTAMP NOT NULL,
    locked_at TIMESTAMP NOT NULL, locked_by VARCHAR(255) NOT NULL);

Add dependency

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-jdbc-template</artifactId>
    <version>4.12.0</version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.jdbctemplate.JdbcTemplateLockProvider;

...
@Bean
public LockProvider lockProvider(DataSource dataSource) {
            return new JdbcTemplateLockProvider(
                JdbcTemplateLockProvider.Configuration.builder()
                .withJdbcTemplate(new JdbcTemplate(getDatasource()))
                .usingDbTime() // Works on Postgres, MySQL, MariaDb, MS SQL, Oracle, DB2, HSQL and H2
                .build()
            );
}

By specifying usingDbTime() (introduced in 4.9.3) the lock provider will use UTC time based on the DB server time. If you do not specify this option, current time on the client will be used (the time may differ between clients).

For more fine-grained configuration use other options of the Configuration object

new JdbcTemplateLockProvider(builder()
    .withTableName("shdlck")
    .withColumnNames(new ColumnNames("n", "lck_untl", "lckd_at", "lckd_by"))
    .withJdbcTemplate(new JdbcTemplate(getDatasource()))
    .withLockedByValue("my-value")
    .withTimeZone(TimeZone.getTimeZone("UTC"))
    .build())

If you need to specify a schema, you can set it in the table name using the usual dot notation new JdbcTemplateLockProvider(datasource, "my_schema.shedlock")

Warning

Do not manually delete lock row from the DB table. ShedLock has an in-memory cache of existing locks so the row will NOT be automatically recreated until application restart. If you need to, you can edit the row/document, risking only that multiple locks will be held. Since 1.0.0 you can clean the cache by calling clearCache() on LockProvider.

Mongo

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-mongo</artifactId>
    <version>4.12.0</version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.mongo.MongoLockProvider;

...

@Bean
public LockProvider lockProvider(MongoClient mongo) {
    return new MongoLockProvider(mongo.getDatabase(databaseName))
}

Please note that MongoDB integration requires Mongo >= 2.4 and mongo-java-driver >= 3.7.0

Reactive Mongo

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-mongo-reactivestreams</artifactId>
    <version>4.12.0</version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.mongo.reactivestreams.ReactiveStreamsMongoLockProvider;

...

@Bean
public LockProvider lockProvider(MongoClient mongo) {
    return new ReactiveStreamsMongoLockProvider(mongo.getDatabase(databaseName))
}

Please note that MongoDB integration requires Mongo >= 4.x and mongodb-driver-reactivestreams 1.x

DynamoDB

This depends on AWS SDK v1.

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-dynamodb</artifactId>
    <version>4.12.0</version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.dynamodb.DynamoDBLockProvider;

...

@Bean
public LockProvider lockProvider(com.amazonaws.services.dynamodbv2.document.DynamoDB dynamoDB) {
    return new DynamoDBLockProvider(dynamoDB.getTable("Shedlock"));
}

Please note that the lock table must be created externally. DynamoDBUtils#createLockTable may be used for creating it programmatically. A table definition is available from DynamoDBLockProvider's Javadoc.

DynamoDB 2

This depends on AWS SDK v2.

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-dynamodb2</artifactId>
    <version>4.12.0</version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.dynamodb2.DynamoDBLockProvider;

...

@Bean
public LockProvider lockProvider(software.amazon.awssdk.services.dynamodb.DynamoDbClient dynamoDB) {
    return new DynamoDBLockProvider(dynamoDB, "Shedlock");
}

Please note that the lock table must be created externally. DynamoDBUtils#createLockTable may be used for creating it programmatically. A table definition is available from DynamoDBLockProvider's Javadoc.

ZooKeeper (using Curator)

Import

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-zookeeper-curator</artifactId>
    <version>4.12.0</version>
</dependency>

and configure

import net.javacrumbs.shedlock.provider.zookeeper.curator.ZookeeperCuratorLockProvider;

...

@Bean
public LockProvider lockProvider(org.apache.curator.framework.CuratorFramework client) {
    return new ZookeeperCuratorLockProvider(client);
}

By default, nodes for locks will be created under /shedlock node.

Redis (using Spring RedisConnectionFactory)

Import

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-redis-spring</artifactId>
    <version>4.12.0</version>
</dependency>

and configure

import net.javacrumbs.shedlock.provider.redis.spring.RedisLockProvider;
import org.springframework.data.redis.connection.RedisConnectionFactory;

...

@Bean
public LockProvider lockProvider(RedisConnectionFactory connectionFactory) {
    return new RedisLockProvider(connectionFactory, ENV);
}

Redis lock provider uses classical lock mechanism as described here which may not be reliable in case of Redis master failure.

If you are still using Spring Data Redis 1, import special lock provider shedlock-provider-redis-spring-1 which works around issue #105 or upgrade to Spring Data Redis 2 or higher.

Redis (using Jedis)

Import

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-redis-jedis</artifactId>
    <version>4.12.0</version>
</dependency>

and configure

import net.javacrumbs.shedlock.provider.redis.jedis.JedisLockProvider;

...

@Bean
public LockProvider lockProvider(JedisPool jedisPool) {
    return new JedisLockProvider(jedisPool, ENV);
}

Hazelcast

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <!-- Hazelcast < 4 -->
    <artifactId>shedlock-provider-hazelcast</artifactId>
    <!-- Hazelcast 4 -->
    <!-- <artifactId>shedlock-provider-hazelcast4</artifactId> -->
    <version>4.12.0/version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.hazelcast.HazelcastLockProvider;

...

@Bean
public HazelcastLockProvider lockProvider(HazelcastInstance hazelcastInstance) {
    return new HazelcastLockProvider(hazelcastInstance);
}

For Hazelcast 4 use shedlock-provider-hazelcast4 module and net.javacrumbs.shedlock.provider.hazelcast4 package.

Couchbase

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-couchbase-javaclient</artifactId>
    <version>4.12.0/version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.couchbase.javaclient.CouchbaseLockProvider;

...

@Bean
public CouchbaseLockProvider lockProvider(Bucket bucket) {
    return new CouchbaseLockProvider(bucket);
}

Elasticsearch

I am really not sure that it's a good idea to use Elasticsearch as a lock provider. But if you have no other choice, you can. Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-elasticsearch</artifactId>
    <version>4.12.0</version>
</dependency>

Configure:

import static net.javacrumbs.shedlock.provider.elasticsearch.ElasticsearchLockProvider;

...

@Bean
public ElasticsearchLockProvider lockProvider(RestHighLevelClient highLevelClient) {
    return new ElasticsearchLockProvider(highLevelClient);
}

CosmosDB

CosmosDB support is provided by a third-party module available here

Cassandra

Import the project

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-provider-cassandra</artifactId>
    <version>4.12.0/version>
</dependency>

Configure:

import net.javacrumbs.shedlock.provider.cassandra.CassandraLockProvider;

...

@Bean
public CassandraLockProvider lockProvider(CqlSession cqlSession) {
    return new CassandraLockProvider(cqlSession);
}

Example for creating default keyspace and table in local Cassandra instance:

CREATE KEYSPACE shedlock with replication={'class':'SimpleStrategy', 'replication_factor':1} and durable_writes=true;
CREATE TABLE shedlock.lock (name text PRIMARY KEY, lockUntil timestamp, lockedAt timestamp, lockedBy text);

Please, note that CassandraLockProvider uses Cassandra driver v4, which is part of Spring Boot since 2.3.

Multi-tenancy

If you have multi-tenancy use-case you can use a lock provider similar to this one (see the full example)

private static abstract class MultiTenancyLockProvider implements LockProvider {
    private final ConcurrentHashMap<String, LockProvider> providers = new ConcurrentHashMap<>();

    @Override
    public @NonNull Optional<SimpleLock> lock(@NonNull LockConfiguration lockConfiguration) {
        String tenantName = getTenantName(lockConfiguration);
        return providers.computeIfAbsent(tenantName, this::createLockProvider).lock(lockConfiguration);
    }

    protected abstract LockProvider createLockProvider(String tenantName) ;

    protected abstract String getTenantName(LockConfiguration lockConfiguration);
}

Duration specification

All the annotations where you need to specify a duration support the following formats

Micronaut integration

Since version 4.0.0, it's possible to use Micronaut framework for integration

Import the project:

<dependency>
    <groupId>net.javacrumbs.shedlock</groupId>
    <artifactId>shedlock-micronaut</artifactId>
    <version>4.12.0</version>
</dependency>

Configure default lockAtMostFor value (application.yml):

shedlock:
  defaults:
    lock-at-most-for: 1m

Configure lock provider:

@Singleton
public LockProvider lockProvider() {
    ... select and configure your lock provider
}

Configure the scheduled task:

@Scheduled(fixedDelay = "1s")
@SchedulerLock(name = "myTask")
public void myTask() {
    assertLocked();
    ...
}

Locking without a framework

It is possible to use ShedLock without a framework

LockingTaskExecutor executor = new DefaultLockingTaskExecutor(lockProvider);

...

Instant lockAtMostUntil = Instant.now().plusSeconds(600);
executor.executeWithLock(runnable, new LockConfiguration("lockName", lockAtMostUntil));

Modes of Spring integration

ShedLock supports two modes of Spring integration. One that uses AOP proxy about scheduled method (PROXY_METHOD) and one that proxies TaskScheduler (PROXY_SCHEDULER)

Scheduled Method proxy

Since version 4.0.0, the default mode of Spring integration is an AOP proxy around the annotated method.

The main advantage of this mode is that it plays well with other frameworks that want to somehow alter the default Spring scheduling mechanism. The disadvantage is that the lock is applied even if you call the method directly. If the method returns a value and the lock is held by another process, null or an empty Optional will be returned (primitive return types are not supported).

Final and non-public methods are not proxied so either you have to make your scheduled methods public and non-final or use TaskScheduler proxy.

Method proxy sequenceDiagram

TaskScheduler proxy

This mode wraps Spring TaskScheduler in an AOP proxy. It can be switched-on like this (PROXY_SCHEDULER was the default method before 4.0.0):

@EnableSchedulerLock(interceptMode = PROXY_SCHEDULER)

If you do not specify your task scheduler, a default one is created for you. If you have special needs, just create a bean implementing TaskScheduler interface and it will get wrapped into the AOP proxy automatically.

@Bean
public TaskScheduler taskScheduler() {
    return new MySpecialTaskScheduler();
}

Alternatively, you can define a bean of type ScheduledExecutorService and it will automatically get used by the tasks scheduling mechanism.

TaskScheduler proxy sequence diagram

Spring XML configuration

Spring XML configuration is not supported as of version 3.0.0. If you need it, please use version 2.6.0 or file an issue explaining why it is needed.

Lock assert

To prevent misconfiguration errors, you can assert that the lock works by using LockAssert:

@Scheduled(...)
@SchedulerLock(..)
public void scheduledTask() {
    // To assert that the lock is held (prevents misconfiguration errors)
    LockAssert.assertLocked();
    // do something
}

In unit tests you can switch-off the assert by calling LockAssert.TestHelper.makeAllAssertsPass(true) on given thread.

Kotlin gotchas

The library is tested with Kotlin and works fine. The only issue is Spring AOP which does not work on final method. If you use @SchedulerLock with @Scheduled annotation, everyting should work since Kotling Spring compiler plugin will automatically 'open' the method for you. If @Scheduled annotation is not present, you have to open the method by yourself.

Caveats

Locks in ShedLock have expiration time which leads to following possible issues.

  1. If the task runs longer than lockAtMostFor, the task can be executed more than once
  2. If the clock difference between two nodes is more than lockAtLeastFor or minimal execution time the task can be executed more than once.

Troubleshooting

Help, ShedLock does not do what it's supposed to do!

  1. Check the storage. If you are using JDBC, check the ShedLock table. If it's empty, ShedLock is not properly configured. If there is more than one record with the same name, you are missing a primary key.
  2. Use ShedLock debug log. ShedLock logs interesting information on DEBUG level with logger name net.javacrumbs.shedlock. It should help you to see what's going on.
  3. For short-running tasks consider using lockAtLeastFor. If the tasks are short-running, they can be executed one after each other, lockAtLeastFor can prevent it.
  4. If you encounter weird error complaining that a Proxy is not class of ThreadPoolTaskScheduler please check https://github.com/lukas-krecan/ShedLock/issues/115 or this StackOverflow quesiton

Requirements and dependencies

Release notes

4.12.0

4.11.1

4.10.1

4.9.3

4.9.2

4.9.1

4.9.0

4.8.0

4.7.1

4.7.0

4.6.0

4.5.2

4.5.1

4.5.0

4.4.0

4.3.1

4.3.0

4.2.0

4.1.0

4.0.4

4.0.3

4.0.2

3.0.1

3.0.0

2.6.0

2.5.0

2.4.0

2.3.0

2.2.1

2.2.0

2.1.0

2.0.1

1.3.0

1.2.0

1.1.1

1.1.0

1.0.0

0.18.2

0.18.1

0.18.0

0.17.0

0.16.1

0.16.0

0.15.1

0.15.0

0.14.0

0.13.0

0.12.0

0.11.0

0.10.0

0.9.0

0.8.0

0.7.0

0.6.0

0.5.0

0.4.1

0.4.0

0.3.0